I propose the following term: Extreme Dummy.

Dummy here is used as it’s used by the publishers of the Dummies books. They’re not really suggesting that their customers are stupid; it’s just that while you might know something about, say, high finance, you can still be totally clueless — a ‘dummy’ — about other things.

Everyone’s a dummy about most things, and the Dummies series fills a real need: most books about a given topic assume a certain familiarity or even expertise with the material, and until the rise of the Dummies phenomenon, the novice had the choice of following a course of expensive and lengthy formal training, or of jumping into the topic in medias res and struggling for a while until he figured out what was going on.

The Dummies books provide a good introduction to a now vast number of topics, giving the reader the necessary background to be able to evaluate other books, formulate Google searches, etc. in order to learn more.

An Extreme Dummy — XD for short — is someone who cannot or will not be helped by the Dummies books or by other methods of foundational instruction in a given field. The Platonic XD reads the Dummies book on a given topic and is just as helpless afterwards as before. Extreme Dummies are usually people who have concluded that they don’t understand a thing because understanding is impossible, and so no amount of education or training will make any difference.